Sternum or Breast Bone
The organs in the thoracic cavity comprise of many critical organs of human body including the heart and lungs. This is the second largest cavity in the body and is surrounded (and protected) by the rib cage, breast bone (sternum) and the vertebral column. 
The sternum, more commonly known as breastbone, is a long, flat bone which has an appearance that resembles the capital letter ‘T’ . It is situated beneath the center of chest, and it measures around 17cm long in average. It primarily functions as a protection to the organs in the thoracic cavity, and it also acts as a joining point for the shoulder bones and ribs .
Picture 1: Location of the sternum
Image source: chw.org
The sternum is subdivided into 3 parts :
- Manubrium – the upper and broader part of breast bone wherein it articulates the upper two ribs and the collar bones that are attached
- Gladiolus or sternum body – the longest portion of the sternum
- Xiphoid process –the small pointed cartilaginous process at the inferior portion of the sternum
Picture 2: Anatomy of the sternum
Image source: courses.vcu.edu
Sternum pain is a sharp, usually unexpected pain in the chest area. The pain is sometimes associated with myocardial infarction (MI) or heart attack because of the similar sharp pain felt by the sufferer. Though it is usually not a serious problem, a chronic pain needs to be referred to a medical practitioner. 
Sternum pain is characterized by a radiating paineither in the inferior side or sides of the breast bone. Mild to moderate sharp and stabbing sensations can be felt. 
In certain cases, the pain is accompanied with tenderness and swelling accompanied by a cracking sound from the sternum joints. Such a condition is referred to as popping sternum. This sound, similar to popping knuckle, jaw or knees, generally originates from the joints connecting the ribs and the sternum. 
Causes of Sternum Pain
A common cause of sternum pain is costochondritis, which is due to a minor injury to the rib cage.Costochondritis is the medical term forpaininthe costochondral cartilage, which is located in the area where the rib cage and sternum meets. Applying excessive force or bruising the chest area may lead to this disorder. In this condition, the chest area may feel tender and sore.
2. Tietze Syndrome
Similar to costochondritis, Tietze syndrome also affects the costochondral cartilage. The difference is that in Tietze syndrome, the costochondral cartilage becomes inflamed. The cause of the inflammation is unknown, but it is usually linked to upper respiratory tract infections, severe cough, chest injury, or strenuous exercise.
3. Joint damage
Joint pains due to extreme physical activities like exercise and weight lifting can indicate a joint damage in the sternum and rib area. The pain felt can be persistent, and can affect the shoulder and collar bones if the cause of joint damage is arthritis.
4. Muscle Pain
The pectoral muscles are attached to the breast bone and any muscle pain involving this group of muscles could lead to a sore or bruised pain in the sternum. Other muscles like the intercostal muscle positioned between the ribs also lead to pain in the sternum. This is usually caused by forced or deep breathing, as in the case of asthma, emphysema and other respiratory disorders.
Picture 3: Muscles around the ribcage area
Image source: teachmeanatomy.com
An accident due to any physical trauma like a car accident can lead to fractures in breast bone. A sternum facture is rare as it can happen only if the forceful blow is felt directly at the breast bone. These fractures can generate mild or severe sternum pain depending on the intensity of the trauma. Fractures involving the sternoclavicular joint and collarbone can also lead to sternum pain.
Any surgery involving the organs in thoracic cavity like an open heart surgery can repeatedly cause breast bone pain even long after the wounds heals. The pain may disappear over time, but may return when triggered stressful activities or changes in weather.
Heartburn or reflux caused by esophagitis leads to a burning pain in the breastbone. Another common cause of severe and burning pain in the sternum includes GERD (gastroesophageal reflux disease). The burning sensation is usually confused with pain experienced during heart attacks.
8. Cardiac Disorders
Some of the serious cardiac disorders causing pain under the breast include angina, myocardial infarction, coronary artery disease, pericarditis and myocarditis.
9. Respiratory Disorders
Aside from asthma and emphysema, other respiratory diseases that can strain the intercostal musclesinclude bronchitis, tracheitis and bronchiectasis. Pneumonia and tuberculosis may also sternum pain in addition to affecting the area around the lungs and sides of chest.
10. Psychological Causes
Various psychological states like anxiety or panic attacks can give rise to pain under the sternum. This can be due to boosted heart rate or palpitations, psychogenic pain or heartburn.
11. Other Conditions
Other uncommon causes of sternum pain include acute pancreatitis, stomach ulcers, lung cancer, lung abscess, upper middle abdominal pain, sarcoidosis, breast cancer and bone cancer. [3, 4, 5, 6]
Diagnosis of Sternum Pain
Usually confused with heart problems, a pain in sternum can be a serious condition if ignored. Early diagnosis of breast bone pain and the appropriate treatment are recommended as it can even lead to severe illnesses like lymphoma and chondrosarcoma.
Various imaging tests like X-ray, CT-scan and MRI can be used to observe any inflammation or injuries in the sternum and rib area. Reviewing the medical history of the patient for past injuries or surgeries is also helpful. 
Treatment of Sternum Pain
The treatment for breastbone pain depends on the duration of the pain. Generally, over-the-counter analgesics are given to alleviate acute pain. For chronic conditions, corticosteroids or NSAIDs are usually prescribed. 
The treatment also depends on the origin or cause of pain. 
- If the sternum pain is caused by anxiety attacks, calming the patient is the first step. Drugs may or may not be needed.
- When the pain is caused by heartburn or any digestive disorders, medications such as antacids are usually given.
- For costochondritis and Tietze syndrome, anti-inflammatory drugs are given to reduce the pain and swelling in the costochondral joint. A hot or cold compress can also reduce the inflammation.
- When there are fractures involved, proper medical attention is needed.
- Pain caused by heart disorders also needs immediate medical attention.
Lastly, avoiding strenuous activities and proper rest can help alleviate sternum pain.